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R. Silva" , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, devel@daynix.com References: <20250319-hybrid-v1-1-4d1ada10e705@daynix.com> <86plidmjwh.wl-maz@kernel.org> <86o6xxmg87.wl-maz@kernel.org> <86msdhmemw.wl-maz@kernel.org> Content-Language: en-US From: Akihiko Odaki In-Reply-To: <86msdhmemw.wl-maz@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2025/03/19 20:41, Marc Zyngier wrote: > On Wed, 19 Mar 2025 11:26:18 +0000, > Akihiko Odaki wrote: >> >> On 2025/03/19 20:07, Marc Zyngier wrote: >>> On Wed, 19 Mar 2025 10:26:57 +0000, >>>> >>> But that'd be a new ABI, which again would require buy-in from >>> userspace. Maybe there is scope for an all CPUs, cycle-counter only >>> PMUv3 exposed to the guest, but that cannot be set automatically, as >>> we would otherwise regress existing setups. >>> >>> At this stage, and given that you need to change userspace, I'm not >>> sure what the best course of action is. >> >> Having an explicit flag for the userspace is fine for QEMU, which I >> care. It can flip the flag if and only if threads are not pinned to >> one PMU and the machine is a new setup. >> >> I also wonder what regression you think setting it automatically causes. > > The current behaviour is that if you don't specify anything other than > creating a PMUv3 (without KVM_ARM_VCPU_PMU_V3_SET_PMU), you get *some* > PMU, and userspace is responsible for running the vcpu on CPUs that > will implement that PMU. When if does, all the counters, all the > events are valid. If it doesn't, nothing counts, but the > counters/events are still valid. > > If you now add this flag automatically, the guest doesn't see the full > PMU anymore. Only the cycle counter. That's the regression. What about setting the flag automatically when a user fails to pin vCPUs to CPUs that are covered by one PMU? There would be no change if a user correctly pins vCPUs as it is. Otherwise, they will see a correct feature set advertised to the guest and the cycle counter working. Regards, Akihiko Odaki > > Thanks, > > M. >